1. Field of the Invention
The present invention relates to improved methods and apparatus for completing wells in unconsolidated subterranean zones, and more particularly, to improved methods and apparatus for achieving a uniform pack during gravel or frac packs in completing such wells whereby the migration of fines and sand with the fluids produced therefrom is prevented.
2. Description of the Prior Art
Oil and gas wells are often completed in unconsolidated formations containing loose and incompetent fines and sand which migrate with fluids produced by the wells. The presence of formation fines and sand in the produced fluids is disadvantageous and undesirable in that the particles abrade and damage pumping and other producing equipment and reduce the fluid production capabilities of the producing zones in the wells.
Unconsolidated subterranean zones are stimulated by creating fractures in the zones and depositing particulate proppant material in the fractures to maintain them in open positions. In addition, the proppant can be consolidated within the fractures into hard permeable masses to reduce the migration of formation fines and sands through the fractures with produced fluids.
Gravel/frac packs, which include sand screens and the like, are commonly installed in the well bores penetrating unconsolidated zones. The gravel packs serve as filters and help to assure that fines and sand do not migrate with produced fluids into the well bores.
In a typical gravel/frac pack completion, a screen is placed in the well bore and positioned within the unconsolidated subterranean zone which is to be completed. The screen is typically connected to a tool which includes a production packer and a cross-over, and the tool is in turn connected to a work or production string. A particulate material, which is usually graded sand, often referred to in the art as gravel, is pumped in a slurry down the work or production string and through the cross over whereby it flows into the annulus between the screen and the well bore. The liquid forming the slurry leaks off into the subterranean zone and/or through the screen which is sized to prevent the sand in the slurry from flowing therethrough. The sand in the slurry has a very high permeability. As the fluid leaks off into the perforations into the formation and back into the screen, the sand is deposited in the annulus around the screen where it forms a gravel pack. The size of the sand in the gravel pack is selected such that it prevents formation fines and sand from flowing into the well bore with produced fluids.
During a gravel-packing operation, it is imperative to pack the gravel in the perforations and along the entire length of the screen. Conventional gravel packing begins at the bottom of the screen and packs upward. However, with a high leak off of fluid through the perforations in the formation, more and more sand is deposited around the perforations thus forming a node around the perforations. A node is a build up of sand which can grow radially so as to form a bridge and completely block the annular area between the screen and well bore. Although the primary flow of the gravel pack slurry is axial, as the nodes around the perforations build, the flow becomes radial due to the sand build up thus causing the nodes to grow radially around the annulus. If permeability variations and/or the hole geometry cause a gravel bridge to form in the annulus around the screen during packing, the gravel slurry will begin packing upward from the bridge. This is particularly a problem in gravel packs in long and/or deviated unconsolidated producing intervals. The resulting incomplete annular pack has sections of screen that remain uncovered, which can lead to formation sand production and eventual failure of the completion.
FIG. 1 illustrates the problem of the formation of sand bridges 50 in the annulus 52 near the middle of the screen 54 resulting in a non-uniform sand packing of the annulus 52 between the screen 54 and the well bore 56. This often occurs as a result of the loss of carrier liquid from the sand slurry into high permeability portions of the subterranean zone 58 which in turn causes the formation of sand bridges 50 in the annulus 52 before all the sand has been placed. The sand bridges block further flow of the slurry through the annulus 52 which leaves voids 60, 62 in the annulus 52. When the well is placed on production, the flow of produced fluids is concentrated through the voids 60, 62 in the gravel pack which soon causes the screen 54 to be eroded and the migration of fines and sand with the produced fluids to result.
In attempts to prevent the formation of sand bridges in gravel pack completions, special screens having internal shunt tubes have been developed and used. See for example U.S. Pat. No. 4,945,991. While such screens have achieved varying degrees of success in avoiding sand bridges, they, along with the gravel packing procedure, are very costly.
Further improved apparatus and methods of preventing sand bridges are shown in U.S. patent application Ser. No. 09/361,714 filed on Jul. 27, 1999, which is a continuation-in-part of application Ser. No. 09/084,906 filed on May 26, 1998, now U.S. Pat. No. 5,934,376, which is a continuation-in-part of application Ser. No. 08/951,936 filed on Oct. 16, 1997, all hereby incorporated herein by reference. See also European patent application EP 0 909 874 A2 published Apr. 21, 1999 and European patent application EP 0 909 875 A2 published Apr. 21, 1999, both hereby incorporated herein by reference. A slotted liner, having an internal sand screen disposed therein, is placed within an unconsolidated subterranean zone whereby an inner annulus is formed between the sand screen and the slotted liner. The inner annulus is isolated from the outer annulus between the slotted liner and the well bore in the zone and provides an alternate flow path for the particulate material. Particulate material is injected into the inner annulus and outer annulus between either or both the sand screen and the slotted liner and the liner and the zone by way of the slotted liner whereby the particulate material is uniformly packed into the annuli between the sand screen and the slotted liner and between the slotted liner and the zone. If a bridge forms in the outer annulus, then the alternate flow path through the inner annulus allow the filling of the void beneath the bridge in the outer annulus. The permeable pack of particulate material formed prevents the migration of formation fines and sand with fluids produced into the well bore from the unconsolidated zone. Sand bridges may still, however, form in both the inner and outer annuli causing voids in the gravel pack.
Thus, there are needs for improved methods and apparatus for completing wells in unconsolidated subterranean zones whereby the migration of formation fines and sand with produced fluids can be economically and permanently prevented while allowing the efficient production of hydrocarbons from the unconsolidated producing zone.
The present invention provides improved methods and apparatus for completing wells, and optionally simultaneously fracture stimulating the wells, in unconsolidated subterranean zones which meet the needs described above and overcome the deficiencies of the prior art. The improved methods basically comprise the steps of placing a slotted liner having an internal sand screen disposed therein with dividers extending between the liner and screen whereby alternative flow paths in an inner annulus are formed between the sand screen and the slotted liner in an unconsolidated subterranean zone, isolating the outer annulus between the slotted liner and the well bore in the zone, injecting particulate material into either or both the flow paths between the sand screen and the slotted liner and the outer annulus between the liner and the zone by way of the slotted liner whereby the particulate material is uniformly packed into the annuli between the sand screen and the slotted liner and between the slotted liner and the zone. The alternate flow paths prevent voids from forming beneath nodes or bridges in either the inner or outer annuli thereby achieving a uniform pack. The permeable pack of particulate material formed prevents the migration of formation fines and sand with fluids produced into the well bore from the unconsolidated zone.
As mentioned, the unconsolidated formation can be fractured prior to or during the injection of the particulate material into the unconsolidated producing zone, and the particulate material can be deposited in the fractures as well as in the annuli between the sand screen and the slotted liner and between the slotted liner and the well bore.
The apparatus is basically comprised of a slotted liner having an internal sand screen assembly disposed therein. The internal sand screen assembly includes a base member and sand screen with dividers or channelizers extending between the slotted liner and sand screen whereby alternative flow paths are formed in the inner annulus between the sand screen and the slotted liner, a cross-over, adapted to be connected to the production string, is attached to the slotted liner and sand screen assembly and a production packer is attached to the cross-over.
The channelizers, extending between the slotted liner and internal sand screen, divide the inner annulus into a plurality of alternate flow paths. Thus, as nodes build across the inner annulus, the channelizers break up the node. Although the node may plug one of the alternative flow paths between adjacent channelizers, the channelizers prevent the node from extending into one of the other alternative flow paths thus preventing the node from becoming a bridge blocking the entire inner annulus.
The improved methods and apparatus of this invention avoid the formation of voids beneath sand bridges in the inner annulus between the slotted liner and sand screen and in the outer annulus between the slotted liner and the well bore thereby producing a very effective sand screen for preventing the migration of fines and sand with produced fluids.
It is, therefore, a general object of the present invention to provide improved methods of completing wells in unconsolidated subterranean zones.